1. Field of the Invention
The present invention relates to an optical apparatus having a visual axis detection device and, more particularly, to an optical apparatus having a visual axis detection device for detecting an axis in a watching point direction, i.e., a so-called visual axis, in which an observer (photographer) observes through a finder system, on an observation surface (focal plane) on which an object image is formed by a photographing system by utilizing a reflected image of an eye obtained when the eye surface of the observer is illuminated with light.
2. Related Background Art
Conventionally, various apparatus (e.g., eye cameras) for detecting a position, where an observer observes, on an observation surface, i.e., a so-called visual axis, have been proposed.
For example, in Japanese Patent Application Laid-Open No. 61-172552, a parallel light beam from a light source is projected onto a front eye portion of an eye of an observer, and a visual axis is obtained by utilizing a cornea reflected image formed by light reflected by a cornea, and a focusing position of a pupil. FIGS. 18A and 18B are explanatory views of the principle of a visual axis detection method. FIG. 18A is a schematic view showing principal part of a visual axis detection optical system, and FIG. 18B is an explanatory view showing the intensity of an output signal from a photoelectric transducer array 6 in FIG. 18A.
In FIG. 18A, a light source 5 comprises, e.g., a light-emitting diode for emitting infrared rays, that cannot be sensed by an observer, and is arranged on the focal plane of a projection lens 3.
Infrared rays emitted from the light source 5 are converted into parallel light by the projection lens 3, and the parallel light is then reflected by a half mirror 2, thus illuminating a cornea 21 of an eye 201. At this time, a cornea reflected image (virtual image) d formed by some infrared rays reflected by the surface of the cornea 21 is focused by a light-receiving lens 4 through the half mirror 2, and is then re-focused at a position Zd' on the photoelectric transducer array 6.
Light components from edge portions a and b of an iris 23 form images of the edge portions a and b at positions Za' and Zb' on the photoelectric transducer array 6 through the half mirror 2 and the light-receiving lens 4. When a rotational angle .theta. as an angle defined between the optical axis (optical axis O.sub.1) of the light-receiving lens 4 and the optical axis (optical axis O.sub.2) of the eye is small, if the z-coordinates of the edge portions a and b of the iris 23 are represented by Za and Zb, a coordinate Zc of the central position c of a pupil 24 is given by: EQU zc.congruent.(Za+Zb)/2
Since the z-coordinate of the cornea reflected image d coincides with the z-coordinate of a center of curvature O of the cornea 21, if the z-coordinate of a generation position d of the cornea reflected image is represented by Zd, and a distance from the center of curvature O of the cornea 21 to the center C of the pupil 24 is represented by L.sub.OC, the rotational angle .theta. as an angle defined between the eye optical axis O.sub.2 and the optical axis O.sub.1 substantially satisfies the following relation: EQU L.sub.OC * SIN .theta..congruent.Zc-Zd (1)
For this reason, an arithmetic means 9 can obtain the rotational angle .theta. of the optical axis O.sub.2 of the eye 201 by detecting singular points (the cornea reflected image d and the edge portions a and b of the iris) projected onto the surfaces of the photoelectric transducer array 6, as shown in FIG. 18B. At this time, relation (1) is rewritten as: ##EQU1## where .beta. is the magnification determined by a distance L1 between the generation position d of the cornea reflected image and the light-receiving lens 4, and a distance L0 between the light-receiving lens 4 and the photoelectric transducer array 6.
The optical axis O.sub.2 of the eye of the observer does not coincide with the visual axis. U.S. application Ser. No. 671,656 discloses a technique for detecting a visual axis by performing angular compensation between the optical axis of an eye of an observer, and the visual axis. In this technique, a horizontal rotational angle .theta. of the optical axis of the eye of the observer is calculated, and when an angular compensation value between the optical axis of the eye and the visual axis is represented by .delta., a horizontal visual axis .theta.H of the observer is calculated as: EQU .theta.H=.theta..+-..delta. (3)
As for the sign .+-., if the clockwise rotational angle in association with the observer is assumed to be a positive angle, when the eye of the observer at an observation apparatus is his or her left eye, the sign "+" is selected; otherwise, the sign "-" is selected.
FIG. 18A exemplifies a case wherein the eye of the observer is rotated within the Z-X plane (e.g., the horizontal plane). The same applies to a case wherein the eye of the observer is rotated within the X-Y plane (e.g., the vertical plane).
In this case, since the vertical component of the visual axis of the observer coincides with a vertical component .theta.' of the optical axis of the eye, a vertical visual axis .theta.V is given by: EQU .theta.V=.theta.' (4)
FIG. 19 is a schematic view of principal part of an optical system when the visual axis detection device shown in FIG. 18 is applied to a portion of a finder system of a single-lens reflex camera.
In FIG. 19, object light transmitted through a photographing lens 101 is reflected by a quick return mirror 102, and is focused near the focal plane of a focusing screen 104. The object light diffused by the focusing screen 104 becomes incident on an eye point 201a of a photographer through a condenser lens 105, a pentagonal prism 106, and an eye-piece lens 1 having a light splitting surface 1a.
A visual axis detection optical system is constituted by an illumination means (optical axis O.sub.3) including the light source 5 comprising, e.g., an infrared light-emitting diode, that cannot be sensed by a photographer (observer), and the projection lens 3, and a light-receiving means (optical axis O.sub.1) including the photoelectric transducer array 6, the half mirror 2, and the light-receiving lens 4. The optical system is arranged above the eye-piece lens 1 having the light splitting surface 1a comprising a dichroic mirror. Infrared rays emitted from the infrared light-emitting diode 5 are reflected by the light splitting surface 1a, and illuminate an eye an 201 of a photographer. Furthermore, some infrared rays reflected by the eye 201 are reflected by the light splitting surface 1a again, and are then focused on the photoelectric transducer array 6 through the light-receiving lens 4 and the half mirror 2. The arithmetic means 9 calculates the direction of visual axis of the photographer on the basis of image information of the eye (e.g., an output signal shown in FIG. 18B) obtained on the photoelectric transducer array 6. More specifically, a point (visual point) on the focusing screen 104 observed by the observer is obtained.
In this case, the position (Zn, Yn) on the focusing screen 104 observed by the observer is calculated as follows on the basis of the horizontal and vertical visual axes .theta.H and .theta.V described above. ##EQU2## where m is a constant determined by the finder system of the camera.
When the position, observed by the photographer, on the focusing screen 104 in the single-lens reflex camera can be detected in this manner, the following effect can be obtained. That is, when focusing points are arranged not only at the center of the screen but also at a plurality of positions on the screen in, e.g., an automatic focus detection device of a camera, and when a photographer selects one of these points to perform focus detection, an operation for selecting and inputting one point can be omitted, and the point observed by the photographer, i.e., a watching point, is regarded as a point to be subjected to focus detection. Thus, the point can be automatically selected to perform automatic focus detection.
In general, many people use cameras irrespective of age or sex, and photographers using the cameras have different size l of eyes. In the above-mentioned visual axis detection method, relation (2) for calculating the rotational angle .theta. of an eye includes a parameter L.sub.OC (the distance from the center of curvature O of the cornea 21 to the center C of the pupil 24) associated with the size of the eye. For this reason, when the size of the eye of a person who uses a camera, i.e., the parameter L.sub.OC is considerably offset from a value corresponding to the predetermined distance L.sub.OC, the calculated rotational angle .theta. of the eye becomes different from an actual rotational angle of the eye, thus impairing visual axis detection precision.
Furthermore, the compensation angle .delta. between the optical axis of the eye and the visual axis in equation (3) also varies depending on characteristics such as the size of the eye of the photographer. For this reason, when the compensation angle is set to be a constant value, a difference between the calculated direction of visual axis .theta.H and an actual direction of visual axis is generated depending on photographers, thus also impairing visual axis detection precision.
In a commercially available eye camera for measuring the visual axis, a personal differences of different users is compensated for. However, since the optical axis of the eye of the user does not coincide with the optical axis of a camera for photographing a landscape that is supposed to be seen by the user, a target observed by the user must be separated from the eye camera, and the target cannot be integrated with the eye camera.
Furthermore, in order to adjust the eye camera, so that the position of the target photographed by the camera and displayed on a television monitor coincides with the position of the visual axis detected when the observer watches the target, an experimental assistant is needed, resulting in the persing of a cumbersome adjustment.